A topological analysis of the electron density in cation-pi complexes is presented. Calculations an performed for complexes involving the interaction of the Na+ ion with a variety of six-and five-membered aromatic rings, as well as for the interaction of benzene with diverse cations and hydrogen-bonded neutral molecules. The results reveal the existence of great differences in the topological features of the electron density, which reflect the profound influence of the substituents attached to the aromatic core or of the presence of heteroatoms and fused rings. Nevertheless, with exception of the complexes involving five-membered rings, there are strong relationships between the charge density at the cage critical points and both the energetic and geometrical parameters of the complexes. These relationships allow us to generalize the concept of bond order-bond length found for other intermolecular interactions.